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RE: [linux-lvm] snapshots of busy ext2 file system corrupt



On Wed, 27 Feb 2002, Chris Mason wrote:

> >> I think, the VFS patch has some principal problems. Creating 
> >> a snapshot
> >> with the VFS-lock patch applied is more or less equivalent to 
> >> unmounting 
> >> the file system, creating the snapshot of the device and 
> >> remounting the
> >> file system. That means that all ongoing write operations must be
> >> suspended until the filesystem is in a "clean" state. This can take
> >> some time. Up to 15 minutes from my observations and that is 
> >> way too long.
> >> I think the right way is: use a jornaling file system, take a 
> >> snapshot,
> >> make the snapshot writeable, replay the log, make the 
> >> snapshot readonly
> >> and dump it to tape or whatever you want. No races, no deadlocks, no
> >> problems.
> 
> There is a problem with this method, namely the FS you get after
> replaying the log will not be the same FS you had at the time of
> the snapshot.  Any transactions that had not been committed yet will
> not be replayed.  It will be consistent, but probably not the FS you
> were expecting.

It depends. For backups triggerd by cron that is not an issue, because i
only expect a snapshot of the filesystem at about 2am. For other purposes
it might be a problem. Would a call to sync prior to the lvcreate -s help?

Anselm


---
 Anselm Kruis                                 Tel. +49 (0)89-356386-74
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